What is a back mouse?

Back Mice: Definition and Clinical Significance

A back mouse (episacroiliac lipoma) is a subcutaneous fibro-fatty nodule typically found in the lumbosacral region that can be a treatable cause of low back pain and referred pain patterns. These nodules are firm, rubbery, mobile masses that occur in characteristic regions including the sacroiliac, posterior superior iliac, and lumbar paraspinal areas 1.

Characteristics of Back Mice

• Back mice represent herniated fatty tissue that has protruded through fascial layers, creating palpable subcutaneous nodules 1
• They are found in up to 16% of the general population 2
• These nodules are typically located near the posterior superior iliac spine and lumbosacral region 3
• Back mice are characterized as firm, rubbery, and mobile upon palpation 1

Clinical Presentation

• Patients may present with localized low back pain directly over the nodule 4
• Pain can radiate to distant locations including the lower leg (37% of cases), lower abdomen, or inguinal region 5, 4
• Some patients (14%) experience leg numbness or paresthesias that can mimic radicular symptoms 5
• The duration of pain before diagnosis varies widely, from a few weeks to several years (median 8 weeks) 5
• Point tenderness over the nodule is a characteristic finding that may be confused with myofascial pain 1

Diagnostic Approach

• Diagnosis is primarily clinical, based on the presence of palpable, tender nodules in characteristic locations 1
• Sonographic features include subfascial fat herniation visible on ultrasound imaging 3
• Key distinguishing features from other causes of back pain include:
  • Absence of neurological deficits 1
  • Reproduction of pain with direct palpation of the nodule 1
  • Characteristic location near the posterior superior iliac spine 3

Treatment Options

• Injection therapy with local anesthetic and corticosteroid has shown effectiveness in 89% of patients in case series 5
• Multiple puncture or needling of the nodule may be effective by reducing tension within the fibro-fatty nodule 4
• Treatment often relieves both local and radiating symptoms, though it may not eliminate the physical nodule itself 5
• Patient satisfaction with injection treatment is high (86% reported being "satisfied" or "very satisfied") 5
• Relief from injection therapy can be long-lasting with no significant adverse events reported 5

Differential Diagnosis

• Back mice may be confused with:
  • Herniated disc due to similar radicular symptoms 1
  • Myofascial pain syndrome due to point tenderness 1
  • Trochanteric bursitis when pain refers to lateral hip 4
  • Lower abdominal pain conditions when pain refers anteriorly 4

Clinical Significance

• Back mice represent an under-recognized but potentially treatable cause of low back pain 1
• Identifying these nodules can prevent unnecessary expensive diagnostic testing and interventions 5
• Physicians should consider this diagnosis in patients with unexplained low back pain, especially when accompanied by distant referred pain patterns 4

Back mice should be considered in the differential diagnosis for patients presenting with atypical low back pain patterns, particularly when standard imaging and treatments have failed to provide relief 4, 1.